Various devices have been designed and manufactured over time and which are useful when used in an earthen profile to determine or calculate hydraulic gradients. These hydraulic gradients have been employed to determine the direction of water movement and to estimate water flux using unsaturated hydraulic conductivity. As should be understood, the movement of water in an unsaturated earthen zone is important for engineering studies, hazardous waste site monitoring, recharge studies and irrigation management practices. For example, if the moisture potential of soil can be accurately monitored, irrigation can be controlled to optimize the rate of plant growth.
One type of instrument used heretofore for measuring soil moisture potential is the tensiometer. A conventional tensiometer comprises a sealed tube defining a chamber which is normally completely filled with water; a hollow porous tip on one end of the tube; and a vacuum gauge connected to the water chamber. The porous tip is inserted in the soil and establishes liquid contact between the water in the tube and the moisture in the soil surrounding the tip. Relatively dry soil tends to pull water from the tube through the porous tip. However, since the tube is sealed, only a minute amount of water is actually withdrawn. Accordingly, the water in the tube is placed under tension by the pulling effect of the dry soil, thus creating a measurable subatmospheric pressure in the tube. Higher moisture contents in the soil produce correspondingly less vacuum in the tube, and completely saturated soils register substantially zero vacuum or atmospheric pressure.
Conventional tensiometers have been installed, generally, within a few meters of the land surface because the length of the water column employed with same will determine, to some degree, the accuracy of the tensiometer. In this regard, it should be understood that there is a physical limit to the length of the column of water which can be supported by atmospheric pressure (about 1,020 centimeters at sea level) and the useful measurement range of the tensiometer is reduced as the column of water above the porous tip is lengthened. In this regard, the pressure exerted by the column of water increases the pressure in the porous tip, which in turn increases the apparent soil moisture tension recorded by the above-surface pressure measuring devices employed with same.
Conventional tensiometers may be constructed with pressure transducers buried at or near the sensing tip to circumvent this depth limitation and allow automated data collection. While this design operates with some degree of success, it has shortcomings which have detracted from its usefulness. For example, this design does not allow for periodic calibration of the transducer, replacement of the transducer, or refilling of the instrument. Others skilled in the art have attempted to avoid this perceived shortcoming by designing air filled tensiometers and utilizing various measurement practices to address the shortcomings associated with same. All these practices have met with limited success.
There remains a need, therefore, for a monitoring well which can be utilized in combination with various geophysical, and hydrogeologic monitoring devices and which is operable to measure various soil parameters such as moisture potential deep within sub-grade earthen soil. Although the principal motivation for this invention arose from concerns associated with deep soil use of tensiometers, those artisans skilled in this field will recognize other inventive uses of the invention which is only to be limited by the accompanying claims appropriately interpreted in accordance with the Doctrine of Equivalents.